Heat treating A-2 tool steel

[Juan Hovey]

Hey, folks: I'm thinking about making my own A-2 irons for my infill hand planes, and I have a few questions.

1. I've read varying accounts on heat-treating A-2 tool steel as distinct from O-1. All say you must plunge O-1 into oil after soaking it at the proper temperature, but when it comes to heat-treating A-2, some say plunge it into oil and some say let it cool in air. Which is correct?

2. Accounts vary as regards tempering the steel, too. Most say you must temper the steel directly after heat-treating it, but I've also read that you can re-temper steel. If you can re-temper it, what's the rush to temper it in the first place?

3. Last but not least, why heat-treat the entire iron? Put another way, why not heat-treat only the business end? I've done some experimenting with a torch and a make-shift chamber formed out of fire brick, and I've had no trouble getting the business end of several test irons to the point where the color "blooms," or suddenly changes from red to bright orange. The chamber was narrow, making it impossible to treat the entire length of the iron - which would be hard to do in any case with a torch. But the irons sharpened up nicely after plunging them in oil and then tempering them in a reliable oven, and it occurred to me that I might not need to worry about heat-treating the entire thing. Am I wrong?

[george wilson]

Here's the straight poop,Juan. I use A2 frequently: You must protect the A2 from air when it is at hardening temperature.Hardening temp. is 1750º F. If you don't,it will have a soft(decarbed) skin on it that can be 1/32" deep. It will be RUINED. What furnace are you using? There is only a 25 degree "window" of getting just the proper heating temperature to the A2. You need a furnace with an ACCURATE pyrometer.

Wrap the A2 in HIGH TEMP stainless wrap. Put a SMALL(1/2 postage stamp size) piece of brown paper in the envelope to burn out any oxygen still in it. The seams must be double crimped to keep air out. Repeat: double crimped. I do this with smooth steel jaws in a machinist's vise. Put in a little too much paper,and the envelope will burst open and ruin the iron inside.
Be careful,that stainless foil WILL cut you pretty bad if you aren't careful. I often get cut myself,to some annoying degree!!! Just handling the stuff is dangerous.

Yes,AIR COOL the A2. Don't let anyone tell you different. A2 is an AIR HARDENING STEEL. Quenching it in any medium will cool it too fast,and could cause warping or cracking,and also screw up the molecular structure of the steel. Read the instructions that you can easily find for heat treating most any steel online. The envelope must be cut open,and the iron removed QUICKLY so it can cool. Prop it up on 2 firebricks to cool. DO NOT lay the iron on metal to cool!!! It will cool TOO FAST. The best thing is to stand it on edge,so both sides can cool the same. STILL AIR is just fine:no fan needed. I just use a pair of scissors dedicated to cutting the hot foil open.

To temper,the iron is best tempered just as it has become cool enough to almost hold in your hand(with a bit of juggling!) I use a toaster oven with an ACCURATE high temperature thermometer with a LONG probe on it. Buy one from Brownell's Gunsmith supply. Find a slot on the back of the toaster to insert the probe into. Make sure you don't touch a live wire in the toaster. I burned a hole in a probe doing that once. Insert the probe when the oven is OFF.

Have the toaster oven pre heated,and immediately put the still quite warm iron into it at 400 degrees. That's 400 degrees. 400º will yield 60 Rockwell C..The foil will be OFF the A2 iron at this stage. You cut it off previously. Let it heat for an hour. Then,and only then can you let the iron cool off.

Follow these directions properly,and all will be well. If you find a soft skin on your iron after heat treating,it means you did not get the envelope properly double crimped,and some air got into it. I cannot over stress sealing the envelope. Fold the seam,pressing it flat with the vise. Then,half way across the folded seam,fold it again,and squeeze the seam dead flat again,with the vise jaws. The seam will be 4 layers thick at the seams. Make the seams along the SIDES of the iron,not in the middle,where you can't do an effective job. Make the envelope as absolutely snug around the iron as you can,to eliminate air. Don't forget the small piece of BROWN paper(it has no clay in it).

P.S.: Do not grind the bevel until the iron has been hardened and tempered. It can warp if the bevel is pre ground. Never pre bevel ANY iron you are making. W1 and 01 will warp WORSE.

[dan sherman]

but when it comes to heat-treating A-2, some say plunge it into oil and some say let it cool in air. Which is correct?

Listen to George, and disregard any future advise given by a source that says quench A2 in Oil, as they have no clue what they are talking about.

[Steve Voigt]

Juan, regarding (2) and (3):
- you temper right away to prevent fractures from developing. In my old machine shop, I remember a guy heat treating a thin disc right before lunch. He didn't want to temper it right away and lose his lunch break, so he set the part on his bench. Half an hour later, he came back and the part had cleanly broken in two. That is an admittedly extreme example but illustrates what can happen. There is no harm in doing multiple temperings; it is the temperature of the hottest tempering cycle that determines the final temper.
- for the kind of small setup you are talking about, I think you are much better off heat treating only the bottom couple inches. Another anecdote from my machine shop days: I made an O1 iron and hardened the whole thing in a commercial oven. Due to my inexperience, it warped during the quench and I was never able to straighten it. If you only harden the bottom 2 inches, there is much less to warp and the non-hardened portion can easily be bent. Anyway, old laminated irons only have tool steel on the bottom, up to the slot in a double iron.

[Tony Zaffuto]

Listen to George, and disregard any future advise given by a source that says quench A2 in Oil, as they have no clue what they are talking about.

++ to this advice.

The prefix "A" means "air harden"! Heat treat manuals are available from the tool steel suppliers with specific recipes per type of steel and mass of steel. In my manufacturing business we do a lot of heat treating for tooling we build in house, as well as use outside sources for tools built outside or for tools too large to predictably harden.

George also made an interesting comment about D2. One of my competitors made several punches for a tool set, by mistake, from D2. The tools shattered in use, nearly killing the machine operator with the flying shards of steel. Each tool steel is made for a specific use, as far as wearability, shock impact, and so forth.

If someone is interested in posting the characteristics of the steels used in our woodworking tools, PM me and I'll see if I can get a manual to mail out, or at least a scan emailed.

Personally, I don't like A2, much preferring O1 (or W1, though probably available only in vintage tools). For the past several weeks, I've played around with the new LV PM-V11 material, and like it much better than A2, but not yet sure when compared to O1. Finally, does anyone know the composition of Clifton irons?

[george wilson]

I just mentioned in a thread here yesterday,that each steel is made for a specific use. And,that I have a pocket knife that is made of D2,and it will not quite hold a fully sharp edge. It is sharp enough for general stuff,but it will not hold a razor edge.

I have regularly used W1,01,A2,D2 and 4140 steels in my work. I have sometimes used 52100 and ATS34,and a few others,but the former are my regulars.

I'll agree with Tony that 01 would be a better choice for Juan. It will take a lot less trouble to harden,as it does not have to be shielded from the air while at high heat,and it will take a better edge. It won't hold the edge as long as A2,but it will take an initially sharper edge.

People get into thinking that because a steel is more sophisticated,it is better. That is not always true.

[dan sherman]

People get into thinking that because a steel is more sophisticated,it is better. That is not always true.

A2 is better than O1 when it comes to edge retention, but that's about the only thing it's better at.

[Larry Frank]

I have a question about the heat treatment of A2. Given the treatment that George suggests, you still have the issue of retained austenite up to 20% which can cause brittleness. Most commercial operations will use cryogenic treatment to cause the retained austenite to transform to martensite which can then be properly tempered.

How to you avoid the issues with the retained austenite and the loss of properties without the cryogenic treatment? It would seem that without that treatment, you will never develop the full potential of the A2.

[Juan Hovey]

George, Dan, Tony, Steve and Larry - Many thanks. Bottom line, I see, is that the better idea is to have the irons professionally heat treated - and the good news is that, although I live in a farm town, I can have it done locally at a machine shop that does high-end work for several airlines. I'll do some more playing around on my own, but the irons that go into my planes will get professional handling.

[george wilson]

The A2 can be tempered at 500º F to decrease its hardness to 58 Rockwell C hardness,which I personally prefer. I think 60 R.C. is too hard. Everyone seems to want to leave their irons at 60 R.C.,which is definitely pushing the upper limits of hardness vs brittleness. It definitely is doing so in 01 steel.

I have said several times that hardening and tempering steel of any kind is a balancing act. You trade one thing for another no matter what you do. A problem might be trying to explain why your blades are only 58 R.C.,while others make them 60 R.C.. I think a lot of customers might be hardness freaks,who do not understand steel well enough. Again,I will repeat that the best OLD blades(19th. C. ones),were the ones I could BARELY file A LITTLE TEENY BIT,with a new,fine cut Nicholson file(a file of the past,sadly). Use a Bahco. Anyhow,being able to barely file the iron means the blades were more like 54-56 R.C.. A HARD saw blade made of 1095 is 52 R.C.. I mean the GOOD ONES made today by small makers from good 1095 spring steel.

Cryogenic treatment is beneficial,but I'm sure Juan would have to take his irons to a shop where they do that sort of treatment. You just can't keep a ready supply of liquid nitrogen sitting around. And,there are other things to know about it anyway.

In fact,I recommend,if Juan wants to use A2 irons,he gets them professionally heat treated and cryogenically treated also. It costs a little money,but if you send several blades,it will cost less per blade,and Juan is a prolific builder,it seems.

Heat treating any steel,especially when it comes to those that will decarb,requires more and more experience and proper equipment. If you send your blades to a shop where they have INERT GAS furnaces,they will return them nice and clean. If you do it yourself,you will be spending time getting the surfaces free of black oxide. Time is money there also.

You can use the info that your irons are pro treated as a selling point. It is worth some money as a marketing ploy. Except,I wouldn't call it a ploy. That sounds phony,while having this properly is definitely a real benefit.

By the way: Don't forget to MARK your name,etc. on the blades before you harden them!!

I have never sent my own blades out to be treated,but I have a decent setup to deal with the steels I use. If it came to treating HSS,I'd be sending them out myself.

There are plenty of professional knife makers who get their blades hardened and tempered by specialists.

P.S.: Juan,you wrote your post above before I was finished with this one advising you to send the irons out. It is the right decision.

[David Weaver]

Finally, does anyone know the composition of Clifton irons?

IIRC, clifton irons are oil hardening steel.

[Juan Hovey]

George - Good points, one and all. I get my logo onto the snecks of my irons by positioning the die under the plunger of a 1-ton Harbor Freight arbor press and then giving it a good whack with a heavy hammer. Works every time.

Anyway, you're a prince. Many thanks.

FYI, I've done nothing but make one plane after another for the past two and a half years with one goal in mind - that every plane be better than the last - so I'd call myself not so much prolific as just determined. As it happens, though, I'm about to take a break to build a workbench for my son in law and grandson. Don Seawater of Pacific Coast Lumber in nearby San Luis Obispo is milling a very big log of locust for the base and top, and I have a stash of what I think is koa to front the cabinet and drawers that will go into the base. I expect to start work on the bench this week. Fun, fun.

[Kees Heiden]

Hi Juan, do you have a link? I'd like to see some eye candy.

[Juan Hovey]

Kees - My website is www.juanvergara.net. Here are pix of two recent planes. The infill is blue gum eucalyptus on each plane, though from trees grown in different parts of the state.
Juan Vergara Blue gum No. 4 infill handplane.jpg
Juan Vergara No. 4 infill handplane.jpg

[Kees Heiden]

Thank you, I'll go drooling over your site.